This invention relates to a circular polarizer which is formed by arranging a plurality of dielectric plates side-by-side relation to provide an artificial anisotropic medium.
In the case where waveguide circuits heretofore employed for the millimeter wave band, such as a filter and the like, are utilized for frequency bands above 100 GHz, the circuit required therefor is very small and hence difficult to process and, further, the surface resistance of metal increases to cause an abrupt increase in the insertion loss. For use in such frequency bands, a filter utilizing a beam waveguide system has been studied. This kind of filter that has heretofore been proposed is one which employs a selective reflector, a circular polarizer and a Fabry-Perot resonator. The circular polarizer employed for this purpose is such that it is formed by disposing many dielectric plates side by side to present anisotropy equivalently, that is, to make the propagation constants of orthogonal electric field components different from each other. This kind of circular polarizer used in the waveguide is disclosed, for instance, in IRE Trans. Vol. MTT-5, No. 3 (July, 1957), pp. 199-203, Kroschbaum and Chen, "A Method of Producing Broad-Band Circular Polarization Employing an Anisotropic Dielectric", and a circular polarizer usable in the beam waveguide is set forth in U.S. Pat. No. 2,464,269 entitled "Method and Means for Controlling the Polarization of Radiant Energy" issued to Charles G. Smith on Mar. 15, 1949. In these prior art circular polarizers, the ratios b/a and a/.lambda..sub.0 are selected to be sufficiently smaller than 1, a being the distance between the centers of adjacent ones of the dielectric plates, b the thickness of each dielectric plate and .lambda..sub.0 the free space wave length of the plane wave used. As a result of this, the frequency characteristics of the phase difference between the electric field components that are perpendicular and parallel to the dielectric plates, respectively, undergo a linear change having a first-order inclination. Further, the frequency band in which the ratio between major and minor electric fields of the elliptically polarized output wave, this is, the so-called axial ratio, is less than 0.5 dB is only 6% or so in terms of the fractional band width. Accordingly, the band-width is narrow, so that when used as a filter, such circular polarizer cannot serve as a wide-band filter.
An object of this invention is to provide a circular polarizer having a wide frequency band.
Another object of this invention is to provide a wide-band circular polarizer that the frequency characteristics of the phase difference between perpendicular and parallel components present a value of 90.degree. at two different values of a/.lambda..sub.0.
Still another object of this invention is to provide a wide-band circular polarizer which can be formed small.